Process for separating calcium values from magnesium values



PRQ1S EQg-,SJEBARATLIQ l@u cIgM VALUES FROM MAGNESIUM VALUES @ggg-:ogy 29, 1942 I wam? Patented Apr. 24, 1945*' Pitoonss Fon SEPABATING "CALCIUM VALUES FROM MAGNESIUM VALUES George E. Walker, Paoli. Pa...asslnor to Warner Company,Philadelphia, Pa., a corporation of Delaware Application october 29, 1942, serial Namaste s Claims. (ci. 21e-201) lvide a method for the preparation oi relatively pure calcium carbonate or oxide' from impurityuents, including the magnesium compounds.

While the .process of the copendingapplcation isl entirely ofeasible for a batch operation, it reduires careful control and cannot be employed in contaminated materials containing calcium oxide or hydroxide or materials capable of being translformed into calcium oxide or hydroxide, which process insures substantially complete separation of the calciumvalues of the' calcium oxide or hydroxide from the other components of the material treated.

Another object oi the present invention is to Still another object or the invention is to provide a method for'the treatment oi calciumand magnesium-containing materials in which, in the separation of the calcium values 'from the magnesium values. the calcium values vare dissolved and the magnesium velues suspended in the solution of the calcium values are more easily filtered than has heretofore been the case.V

Other objects will be apparent from a consideration o! the specicatlon and claims.

i l In @pending application sena Number 315,851,

'filed January 26, 1940, by JamesA.- Murray, (now Patent No. 2,317,396) there is described and claimed a. Process for separating calcium values from magnesium values, irrwhichm. lime milk is treated with astream of hydrogen sulphide,

the stream being discontinued vas soon as hydro- 'gen sulphide is .evidentin the effluent gas, at

which point all but animmaterial amount of the calcium hydroxide has been converted to calcium hydrosulphide. a'nd substantially none ot the magnesium oxide and/cr hydroxide has 'been converted' v:magnesium hydrosulphide. The solution of calcium hydrosulphide is there-4 after separated from the undissolved constita continuously operable process. Furthermore,

the undissolved material, mainlyf magnesium inous and is, therefore. vdiiilcult to remove Vfrom the calcium hydrosulphide solution by ultratioh. In fact, at times, the iiltering operation is so extremely slow that the process may involve, high filtration costs.. As will hereinafter appear, the process oi the present invention does not require careful control of the cunt of hydrogen "sulphide brought into contact with the material treated, andthe process may be continuously operated .on as large a commercial scale as is desired. In addition, the magnesium oxide and/or hydroxide andV other undissolved material may be rendered more easily removable from the solution of calcium hydrosulphide with the result that the iiltering costs may be rcduced.

In the drawing:

Figure l illustrates a new sheet representative of the process; and

Figure 2 is a diagram showing the concentration relations which exist when a suspension ci calcium hydroxide and magnesium hydroxide and/or oxide is treated with hydrogen sulphide.

The present invention is applicable f or the treatment of a widevariety ol natural andj industrial products containing calcium oidde. or hydroxide or materials capable of being trans- `formed into .those compounds, since practically all natural or industrial products of this type also contain magnesium oxide or hydroxide or materialscapable of being converted intomagnesium oxide or hydroxide. The materials most suited to treatment by the' process are the 'limestones and magnesites. As an example oi. an industrial product suitable forV treatment, the

waste sludge obtained in the production of acetylene from calcium carbide may be cited.

Whatever the starting materiaL'it must contain or be converted into calcium hydroxide and magnesium oxide and/or hydroxide. The process may best be illustrated by its application to dolomitic limestone'-iwhich contains appreciable amounts of magnesium-carbonate. The material may be calcined t0 form dolomltic quicklimo, and the latter maybe slaked or hydrated to form calcium hydroxide and manesium oxide and hydroxide. As is well-known, slaking ot quicklime containing magnesium oxide results in the conversiones sabstantiauyau the herum oxide ystream bf gasv containing carb-on dioxide, for

into calcium hydroxide', but, in view of. the"- greater diioulties encountered in the conversion of magnesium oxide into magnesium hydroxide,

' some magnesium oxide may remain unslaked', de-

pending upon the slaking. conditions. Sofar as the present invention is concerned, the relative amount of theresitiualunsla edmagnesiumoxide 'is immaterial. The separat on of the'calcium `values from'the magnesium values is eected by the use of hydrogen sulphid'epand, as will vhereinafter lbe discussed in detail, it is possible by the process sequence control ofthe present invention to obtain a calcium hydrosulphide solution suJo-V stantially free from magnesium oxide and/orhydroxide and from any other impurities of the material treated which are insoluble in the alkaline hydrosulphide solution. The substantiallyppure calcium. hydrosulphide solution is separated from the insoluble magnesium oxide and/or hydroxide Vand other insoluble impurities by ltration.- The calcium hydrosulphide solul tion is then treated with carbon dioxide gasto precipitate calcium carbonate and to regenerate hydrogen sulphide which is returned to the sulphiding system for reuse. I'he calcium carbonate is ltered from the 'solution and may be distributed to the trade vfor use as a pigment,

iiller-or the like, or may be burned' to form acalcium oxide of unusual purity. The magnesium oxide and/or hydroxide separated from the calciumhydrosulphide solution may be burned to form refractory magnesium oxide or may be otherwise processed. 4

The general principle of the separation process may best be understoodby reference to the flow sheet of Figure 1, wherein the solid lines rep-- resent the flow of liquids and/or solids, and the dotted lines represent the flow of gas.

Dolomitic quicklime and water are added conexample, kiln gas, and the soluble calcium hydrosulphide is converted into the insoluble cal-A c ium carbonate. The carbon dioxide enters absorber I4 and passes consecutively through absorbers I3, I2, II, Ill vand 9, wherein the carbon dioxide is absorbed and hydrogen sulphide is evolved. The hydrogen sulphide leaving absorber v9 is then passed consecutively through absorbers (sulphidors) l, 3 and 2, wherein it is brought into contact with the milk of lime and the hydrogen sulphide is absorbed. The eiiluent liqucr from' the iinal absorber consists of a suspension' of calcium carbonate in water vand is filtered in filter I5 to recover the calcium 'carbonate.

It is to be understood that the number of sulphidors and carbonators mentioned above and shown in the drawing are merely illustrative, and a smaller or larger number may be employed if desired. It is only necessary-to have suicient absorbing equipment to obtain the desired reaction and to insure that the gas leav- Y in@ the iirst absorber in the liquid flow does not contain an appreciableA amount of the gas relied 'upqnto produce the reaction in the. absorbers tinuously to a slakei` I which mayfbe of conventional design. The milk of lime produced therein flows ccnsecutivelythrough a, plurality of gas absorbers-2, 3 and d, termed sulphidors,

-wherein it is treated countercurrently with a gas containing hydrogen sulphide. As will hereinafter appear, sumcient hydrogen sulphide is passed into the absorbers to convert all of the calcium hydroxideinto calcium hydrosulphide,

and atleast a portion of the magnesium oxide and/or hydroxide intomagnesium hydrosulphide.

From the final absorber in the sulphidinglsysi tem, the liquor, now consisting'of a `suspensicm i of the'unconvertedmagne'sium oxide and/or hydroxide andirnpurities-in a solution of calcium of the series.

Since both calcium hydroxide-and magnesium r reactions which exist when a suspension of calcium hydroxide and magnesium oxide and/or hydroxide is treated with hydrogenv sulphide solution. It will be noted that the concentration of dissolved calcium increases steadily until the end point is reached, after which it shows little or no Simultaneously, the concentration of increase. calcium oxide in the magnesium oxide and/or hydroxide decreases steadily until the end point is reached, after which there is little or no further decrease. Prior to the end point, no magnesium oxide and/or hydroxide is found in solution,i but immediately subsequent thereto, magnesiim oxide and/or hydroxide starts to dissolve,

' forming magnesium hydrosulphide.v While operhydrosulphide and magnesium hydrosulphide,

flows to a tank 5 designated the.` Sulphided Slurry Tank which. is provided with a heat source (not shown) 1 so that it may be maintained ata temperature above the decomposition point of magnesium hydrosulphide to convert thisv compoundy into magnesium hydroxide,. in granular form, as will hereinafter be explained. The

:ating as a. batch system, the conditions may be so controlled that 'the addition .of hydrogen sulphide may be stopped as soon as the end point is reached, as described and claimed in copend- Aing application Serial No. 315,851, hereinabove material from tank 5 yis fed to iilter 6. The Aiilter cake c onsists primarily of magnesium hy-l droxide and/or oxide.

-The iiltratel from filter Ii is a clear -liquor containing calcium hydrosulphide in solution and v passes to tank 1, designated as the CMSI-I) z tank,

' from which it ilows through pipe lin'e 8 into the iirst 'of a plurality of absorbers (carbonators) 9, 'and from there. the` liquory and vsusvpended'solids iiow consecutively through absorbers it, II, I2, I3 and i4.. In these absorbers,

the liquid is treated countercurrently with a referred to, this requires accurate control andv is not feasible in a continuously operating process.

In accordance ith the present inventionac.- curate control is not necessary in the sulphiding step-in fact, the sulphiding treatment is-carried beyond the end point to insure complete solubilization of the calcium valuest This re-vlsuits in the'conversion of at least.l a portion of the magnesium oxide and/orl hydroxide into magnesium hydrosulphide, and a subsequent heat treatment, to be further described, 'is relied upon to decompose the magnesium hydrosulphide to form insoluble magnesium hydroxide. Upon filtrationLa substantially magnesiumfreecalcium hydrosulphide solution vis obtained for converl s ,svaese sion into calcium carbonate of a 4high'degree of purity- Referring further to the sulphiding treatment,

i the calcium hydroxide andmagnesiu'm oxideV and/or hydroxide suspended in water ow from ciable amount to avoid the necessity of careful control of the process. In addition, as previously the s laker to the-sulphiding equipment in which it is treated with hydrogen sulphide, advantageously witha, gas comprisinga mixture of hydrogen sulphide and an inert gas, such as nitrogen, ior instance 4a gas containing 0% to 50% hydrogen sulphide. The amount o f calcium hydroxide and magnesium oxide and/or hydroxide` suspended in the water is relatively immaterial,

but. the use of a thin slurry is recommended, for example 80 to 130 grams perliter of solidsin the slurry. It has been found that the sulphiding treatment is advantageously conducted in a plurality of absorbers, preferably three, each of which performs a specific function in the process.

' 'l The first absorber in tneliquid ilow contains milk of lime, substantially unreacted, serving as a safety trap to prevent escape of hydrogen su1.

phide to'the'atm'osphere. Thejmain absorption of-hyd'rogensulphide takes place in the. second absorber, and the third absorber serves to insure complete solubili'zationl of the calc ium lhydroxide and to convert at least a portionloi the magnesium oxide and/or hydroxide into magnesium hydrosulpbide.

No temperature control of the sulphidor's is required, 4and each may be permitted to reach 'theequilibrium of its reaction endotherm. In a soi stated, the magnesium-oxide and/or hydroxide which has been dissolved by the hydrogen. sulphide precipitates in a granular, easily iiltrable.

form upon the subjectionJ to the heating step of the process, and, therefore, 1n order to produce a, more-easi1yiiltrab1e uspension of masnesium hydroxide and impuri ies, it is often desirable to dissolve ,a relatively large amount ot' the magnesium oxide and/or hydroxide. Obviously, the ease ofiiltration of the suspension is increased as the ratio of the precipitated magnesium hydroxide to the other suspended material is increased. 1n certain instances, for example,

when the milk of lime contains a relatively small u amount Voi magnesium oxide and/orI hydroxide,

all of the magnesium oxide and/or hydroxide present may be rendered soluble by the hydrogen sulphide, but usually only'a portion of the magnesium content of the milk of lime treated will be converted to magnesium'hydrosulphide., 'I'he 'amount of magnesium oxide and/or lnrdroxide.

dissolved in any particular operation, will be determined bythe various economic iactors.- As

illustrative iig-lires, the amount of magnesium oxide and/or hydroxide dissolved, figured as MBO, v

may range from 2- or 3 grams per liter or less to 1 0 to 14 grams per liter or more. The sulphided material flows to 'a tank (i,

Fig. 1)' and is heated to decompose the magnesium hydrosulphide and to form granular magtypical case, the temperature ranged from. about 2130? F. when the liquid treatedwas substantially' calcium hydroxide'to about160" F. at .the time offcomplete solubilization ofthe calcium lnvdrqxide. Therrate of ow and the concentratlonfoi the hydrogen sulphide in the gasin any par- 4 ticular case will be selected to obtain the results herein described, and will be Vgoverned by the equipmentemployed, the amount of slaked lime V treated in a given time and the like. For .con- 1 tinuous operation, acontroller may be placed in the das line between the rst and second absorbers, set at a low, but positive, concentrationyof hydrogen sulphide, say 2%, and arranged so that,

when the concentration of hydrogen sulphide at this point 'in the absorption system exceeds that at which the controller is set, a greater flow of milk or lime from the slaker to the' first absorber will taire playa,

ln view of the fact that thereis a continuous ow of `substantially unreacted milkof lime into the second absorber from the iirst and a continuous'flo'w lfrom the second -to the third, there -einsts the 'possibility of a small amount of calcium hydroxide leaving the second .absorber without being reacted. The third absorber, therefore, functions to complete the solution .of any small amount of calcium hydroxide entering from I the4 second absorber and to dissolve at least a por- A tion of the magnesium oxide and/o1" hydromde. At times, oi' coursefthe entire calcium hydroxide content and some-orf the magnesium oxide and/or 4hydroxide maybe rendered soluble in the sec- 0nd absorber.

since .the suipniaing treatment is. carried to.

the point where ymagnesium oxide and/or hydroxlde isiound in the solution, all or the' cali cium. hydroxide has necessarily been converted into calcium -hydrosulphida while, in order to insure-complete solubilizatlon 0I, the calcium hydroxide, it is only necessary to dissolve a very amount of the magnesium oxide and/or hydroxide, it'isde'sirablc -to dissolvean appre- 7 5 phide will be. obtained. The decomposition of the magnesium hydrosulphide is dependen-t on a temperature-time relationship. that is, the lower .the temperature, the longer will be the time of heating required. Temperatures as low as C., or lower, may :be used, but such temperatures require a relatively long time of heating. A temperature oi at least 15 C. or 80 C. is.` usually ern-` ployed, and higher temperatures will reduce the time required for decomposition. The solution may, if desired, be raised to just below the decomposition point of the calcium hydrosulphide, but this is of no advantage, and the use of temperatures between 80 and 90C. is preferred. The calcium' hydrosulphide in the solution does Y not decompose to any appreciable extent .during the heating treatment described, and', therefore,

the calcium content of the magnesium values obtained as a result of the process is not increased thereby. The suspension of magnesium hydroxide is separated from the solution of calcium hy'- drosulphide, i'or example, by filtration.

The clear filtrate of calcium hydrosulphide sol Vlution is processed to recover its calcium values and its sulphide values. The liquor is, therefore, passed countercurrently to carbon. dioxide. gas

. throughs. plurality of as absorbers wherein the l; of carbon dioxide mi an mei/t sen, toi-example, a gas containing 21% to 35%.v

-calclumhydrosulphide' solution'is decomposed by thecarbon dibxide, formingfcalcium carbonate The carbon` dioxide is gaseous as nitroandhydrogen sulphide. advantageously provided by a gas-such carbon dioxide.4 In the case kilngas is available,

it will usually be employed.L -The use of a plu' rality of ii rbers is employed for reasons simi lar to those set forth in the consideration of the si'llpl'iiding system. The reaction between car-2 Y bondioxide and calcium hydrosulplude, however,

is not as rapid as that between hydrogen-sulphide and calcium hydroxide. Hence, more absorber capacity is required, and it has been found that, using a standard comercial gas absorber of good eiciency, five, preferably six absorbers,- in series are desirable. The t absorber in the gas stream acts primarily as a clean-up absorber for the small amounts of calcium hydrosulphide which have not reacted in the other absorbers. The bulk of the' reaction occurs in the second and third absorbers, theremainingabsorbers are employed to remove the small amounts of residual4 carbon dioxide from the gas, so that a` substantially carbondioxide-free 'hydrogen sulphide gas may be returned to the sulphiding system for' usetherein. The susnsion from the last absorber in the liquid Ailow is iiltered or votherwise treated to remove the substantially `pure calcium carbonate therefrom. The rate of ilowand concentration of carbon dioxide in any particular case will 'be selected to obtain the\re sults described, and will be governed by thel equipment employed, the amount .of calcium hydrosulphide in the solution treated in a given time, and the like.

.The process of the present invention may be operated as a batch system, i-f desired,'but it is especially adapted for continuous operation,

either as an entirety or in one or morev of the several steps thereof. When the process'throughout is operated in a continuous manner, the

ouiclxlimey and water is fed continuously to the earnest which comprises `treating an aqueous suspension thereof with hydrogen sulphide and converting the calcium hydroxi e into calcium lnfdrcsulphidc and at least a po on of the esium .values into magnesium hydrosulphide, and heating the said sulphided material to a temperature below the decomposition point of calcium hydrosulphide and decomposing the magnesium -hydrosulphide to precipitate granular magnesium. hydroxide and -to liberate hydrogen sulphide, wl'iereblyA a. solution of calcium hydrosulphide containing substantially all of the calciumvaluespresent as calcium hydroxide in the material treated and substantially free from magnesium values is obtained.

2. The process of separatingcalcium values y f from magnesium values associated therewith in Les slaker which in turn delivers the milk of lime to the first absorber in the liquid dow.\ The liquid and suspension flows continuously through the" sulphidors to the tank in which the solutioxrand suspension arerheated to decompose the magnesium hydrosulphide, the flow through the tank being yadiusted so that the material .will'be retained therein for a 'time suilicient to decompose the -magnesium hydmsulphide. from this tank flows to a continuously operating filter which removes the` magnesium hydroxide andimpurities as the iilter cake. The illtrate consistlngof the calcium hydrosulphide solution passes into a feed box for the carbonators from whichit flows to the first carbonator in the lliquid ilow. The lution with the calcium carbonate in suspension ows through the carbonatorslin the series. and the calcium carbonate is illtered from the solution by :a continuously operating illter.

The material a product resulting from slaking; a mixtureoif the oxides ofcalcium and magnesium which comprises treating' an aqueous suspension thereof with hydrogen sulphide and converting the cab.

'cium hydroxide into-calcium hydrosulphide and at least a, portion of the magnesium values into. magnesiumhydrosulphide, heating the vsaid, sulphided material to a temperature below' the decomposition point ofcalcium hydrosulphide and decomposing the magnesium hydrosulphide to precipitate'granular magnesium: hydroxide and to liberate hydrogen sulphide, Aseparating the calcium hydrosulphide solution fromthe suspended solids, carbonating the said solutionto convert y the calcium hydrosulphide into calcium carbonate" and to liberatehydrogen sulphide, and separat` ing the calcium carbonate from thefsolution.

3. 'I'he lprocess of separating calcium values from magnesium'values associated therewith in a'product resulting from slaking a mixture of the A oxides of Acalcium and magnesium which colu-t prises passing anaqueous suspensicm.A thereof" through an absorption system, passing hydrogen sulphide countercurrently: to said suspension inl said system and converting the calcium hydroxide* 4intocal'cium hydrosulphide and at least afportion of the magnesium values into magnesium hydrosulphide, thesaid sulphided ma'- cium hydrosulphide into calcium carbonate and to The hydrogen sulphide and carbon dioxide eachv pass countercurrently to the liquid `flow. in their respective absorbing systems.

The invention disclosed is operabiewitn a veristy of equipment In particular, whereveria plurallty of absorbers is speciiled, reference is made l more to the separation of the functions of the absorptionA 'operation into itsv component parte rather than to .a speciilctype -ornumber of ma@ chines.

Considerable `modiiication is possible in the Y steps of the process, as well as in the conditions employed in the individualsteps, without departing from the essential features ofthe invention.

1. The steps in the process offseparating cal.

cium values .from magnesium values associated therewith in a product resulting from slaking a.

tionof the magnesium values is converte'd into terial from said absorption system, heating the said sulphded material to a .temperature below. A lthe decomposition point of calcium hydro'sul'-,

an absorption system, carbonating the said solution by passing caribou dioxide countercurrently to said solution in said system to convert the cal-J liberate hydrogen sulphide, substantially free from carbon dioxide, returning the hydrogen sul- -phide to the sul-phiding step of the process, and Separating the calcium carbonate fromthe solu" 131011.'v w v 4. The process of claim 3 wherein only a por--v magnesium-hydrosulphide, wherein the st ilphided material is heated to a temperature between about y 75, C. and about 909 C., and wherein the hydro- 3 gen sulphide liberated' upon the' 'heating of the sulphided material is returned to the sulphiding' -from magnesium values associated thlewith instep of the process.l

5. The' process ofv separating calci values' a product resulting from slaking a mixture of the oxides of calcium and magnesium which com-f` prises` p an aqueous` suspension thereof f mixture oi' the oxides of calcium andmimesium 75 throilzh an absorption system. passing' hydrogen from the calcium hydrosulphide solution, discharging the said sulphided material from said 'absorption system, heating the said sulphided materiall to a temperature below the decomposition point'of calcium hydrosulphide and decomposing the magnesium hydrosulphide to precipitate granular magnesium hydroxide and to liberate hydrogen sulphide, returning the liberated hydrogen sulphide to the sulphiding step of the process, separating the calcium hydrosulphide solution from the suspended solids, passing said solution through an absorption system, carbonating the said solution by passing carbon dioxide countercurrently to said solution in said system to convert the calcium hydrosulphide'into calcium carbonate and to provide hydrogen sulphide substantially free from carbon dioxide; returning the hydrogen sulphide to the sulphiding step Aof the process, and separating the calcium carbonate from the solution.

6. The process of claim wherein the sulphided material is heated to a temperature between about '75 C. and about 90 C. I l

7. The continuously operable process of separating calcium values existing as calcium hydroxide from magnesium values associated therewith in a product resulting from slaking a mixture of the oxides of calcium'and magnesium which comprises continuously passing a suspension thereof' into and through an absorption system, continuously'passing hydrogen sulphide countercurrently to said suspension in said system and converting the calcium hydroxide into calcium `hydros'ulphid'e and at leastga"v portion or the magnesium valuesY into magnesium 'hydlrosulphide, Y ycontinuously discharging the said sulphided material from said absorption system into an apparatus in which-said sulphided material may be heated heating the said sulphided material to a temperature below the decomposition point of a calcium hydroslphide and decomposing the mag- .Y nesium hydrosulphide to precipitate 'granular v magnesium hydroxide and tc liberate hydrogen 'sulphide,retuming the liberated hydrogen su1- phide to the sulphiding step of the process, continuously removing theheated solution `from said apparatus, :continuously iiltering said material to f separate the calcium hydrosulphide solution from the suspended solids, continuously passing the ltrate into and through an absorption system, carbonating the said solution by continuously passing carbon dioxide countercurrently to said solution in saidsystem to convert the calcium hydro- 'sulphide into calcium carbonate and to provide hydrogen sulphide substantially free from carbon dioxide, returning the hydrogen sulphide to the sulphiding step of the process, and continuously filtering the calcium carbonate from thesolution. 8. The process of claim 'l wherein the sulphided material iszheated to a temperature between about GEORGE E. WALKER. 

